FOPEN(3S-SVR4)      RISC/os Reference Manual       FOPEN(3S-SVR4)



NAME
     fopen, freopen, fdopen - open a stream

SYNOPSIS
     #include <stdio.h>

     FILE *fopen (const char *filename, const char *type);

     FILE *freopen (const char *filename, const char *type, FILE
         *stream);

     FILE *fdopen (int fildes, const char *type);

DESCRIPTION
     fopen opens the file named by filename and associates a
     stream with it.  fopen returns a pointer to the FILE struc-
     ture associated with the stream.

     filename points to a character string that contains the name
     of the file to be opened.

     type is a character string beginning with one of the follow-
     ing sequences:

       "r" or "rb"
                open for reading

       "w" or "wb"
                truncate to zero length or create for writing

       "a" or "ab"
                append; open for writing at end of file, or
                create for writing

       "r+", "r+b" or "rb+"
                open for update (reading and writing)

       "w+", "w+b" or "wb+"
                truncate or create for update

       "a+", "a+b" or "ab+"
                append; open or create for update at end-of-file

     The b is ignored in the above types. The b exists to distin-
     guish binary files from text files.  However, there is no
     distinction between these types of files on a UNIX system.

     freopen substitutes the named file in place of the open
     stream.  A flush is first attempted, and then the original
     stream is closed, regardless of whether the open ultimately
     succeeds.  Failure to flush or close stream successfully is
     ignored.  freopen returns a pointer to the FILE structure



                        Printed 11/19/92                   Page 1

FOPEN(3S-SVR4)      RISC/os Reference Manual       FOPEN(3S-SVR4)



     associated with stream.

     freopen is typically used to attach the preopened streams
     associated with stdin, stdout, and stderr to other files.
     stderr is by default unbuffered, but the use of freopen will
     cause it to become buffered or line-buffered.

     fdopen associates a stream with a file descriptor.  File
     descriptors are obtained from open(2), dup(2), creat(2), or
     pipe(2), which open files but do not return pointers to a
     FILE structure stream. Streams are necessary input for
     almost all of the Section 3S library routines.  The type of
     stream must agree with the mode of the open file.  The file
     position indicator associated with stream is set to the
     position indicated by the file offset associated with
     fildes.

     When a file is opened for update, both input and output may
     be done on the resulting stream.  However, output may not be
     directly followed by input without an intervening fflush,
     fseek(3S), fsetpos, or rewind, and input may not be directly
     followed by output without an intervening fseek, fsetpos, or
     rewind, or an input operation that encounters end-of-file.

     When a file is opened for append (i.e., when type is "a",
     "ab", "a+", or "ab+"), it is impossible to overwrite infor-
     mation already in the file.  fseek may be used to reposition
     the file pointer to any position in the file, but when out-
     put is written to the file, the current file pointer is
     disregarded.  All output is written at the end of the file
     and causes the file pointer to be repositioned at the end of
     the output.  If two separate processes open the same file
     for append, each process may write freely to the file
     without fear of destroying output being written by the
     other.  The output from the two processes will be intermixed
     in the file in the order in which it is written.

     When opened, a stream is fully buffered if and only if it
     can be determined not to refer to an interactive device.
     The error and end-of-file indicators are cleared for the
     stream.

SEE ALSO
     close(2), creat(2), dup(2), open(2), pipe(2), write(2),
     fclose(3S), fseek(3S), setbuf(3S), stdio(3S).

DIAGNOSTICS
     The functions fopen and freopen return a null pointer if
     path cannot be accessed, or if type is invalid, or if the
     file cannot be opened.





 Page 2                 Printed 11/19/92

FOPEN(3S-SVR4)      RISC/os Reference Manual       FOPEN(3S-SVR4)



     The function fdopen returns a null pointer if fildes is not
     an open file descriptor, or if type is invalid, or if the
     file cannot be opened.

     The functions fopen or fdopen may fail and not set errno if
     there are no free stdio streams.

     File descriptors used by fdopen must be less than 255.















































                        Printed 11/19/92                   Page 3